What does LED 2 flashing every 5 seconds mean?

Opened my 2017 iMac 21.5 4K to change the HD to SSD. Afterwards it won't boot/power-on. I've checked all the connections and they're good. Checked the diagnostic LEDs and LED1 turns on as soon as it's pugged in, and then LED2 starts flashing on-and-off every 5-seconds. Pressing the power-button does nothing. I measured voltage and button is working fine and so is the power-supply. Changed RAM just in case (since new macs don't have tones, assumed it could be a RAM issue) but nothing changed. Put old drive and that also doesn't work. Any ideas? I've posted this question in other forums and it's been radio-silence :(